With the development of telecommunication technique, the broad wide access system BWA, as the substitute technique of digital subscribe loop DSL and wire transmission technique, is arising people's interest continuously. BWA not only provides services that can compete with the services provided by wire transmission, but also provides services that can not be provided by wire transmission in many situations. Generally speaking, there are two kinds of work situations for BWA providing high speed data service on wireless channel, that is, line of sight (LOS) transmission and none line of sight (NLOS) transmission. The first generation BWA system works in LOS situation, mainly employing single carrier high efficiency modulation technique, such as quad-phase shift key QPSK, quadrature amplitude modulation QAM etc., and decision feedback equalization DFE technique as well as fixed direction antenna technique to overcome the influence of multiple paths and interference, but the disadvantage of this system is in that its coverage area is very small due to the line of sight. The next generation BWA system will operate in NLOS situation, wherein, because without the direct signal component of LOS situation, it will be greatly influenced by multi-path, its mainstream techniques are Orthogonal Frequency Division Multiplexing OFDM technique and multi-antenna technique, so as to overcome the disadvantage of the first BWA system. Currently, the technique standard of the above two situations is drawn by IEEE802.16 broad wide access workgroup.
The first generation BWA system usually uses the frequency division duplex FDD from point to point, in which the uplink uses TDMA, the downlink uses time division multiplexing TDM; the uplink modulation form is QPSK-16QAM, the downlink modulation form is QPSK-64QAM. But currently, the uplink usually uses burst QPSK modulation, because the constellation of QPSK phase modulation is very simple, compared to QAM modulation, the ratio of signal to noise (interference) is lower, so we can employ simple equalization technique (pre-equalization and pre-training technique) or with no equalization, but the disadvantage of QPSK modulation is in that its efficiency of spectrum is very low. In order to meet the increasing requirement of uplink bandwidth for user, the efficiency of spectrum should be improved, because the simple equalization technique could have not meet the requirement when employing 16 QAM modulation and adaptive modulation technique, the burst equalization of BWA system is become a key problem.
In existing BWA system, burst equalization mainly use two kinds of forms, one is pre-equalization technique, and the other is pre-training technique.
Pre-equalization technique: in uplink direction, setting a pre-equalizer (or precoder) on user end, during the ranging process, base station calculates the coefficient of equalizer, and sends the coefficient to user end for pre-equalization process. The idea of this method is to divide multi-path component into static part and dynamic part, in which, static part changes very slowly, and it takes very long time for the coefficient of equalizer to change once, for example, the coefficient will be updated every time the ranging process take place; while the dynamic part changes very quickly, the coefficient will be updated as long as one or several bursts occur, thus, the realization of burst equalization will be simplified. This method can also be employed in cable modem, the 1.1 version of DOCSIS protocol adds the pre-equalization part.
Pre-training technique: before transmitting user data, first sending training sequence; when the coefficient of equalizer is converged enough, transmitting user data; thus, the coefficient of equalizer at the previous burst data ends is memorized, so as to be the equalization of the next burst; once the equalizer can not work due to the change of channel, the training is performed again.
The advantage of the above two kinds of methods is in that the structure of equalizer is simple, and each burst data package does not comprise training sequence, therefore improving the efficiency; but this is only adapted to the situation of static channel or the situation that the interval of burst is very short, once not meeting the above two conditions, the burst equalization is inevitably required. At this time, the front end of each burst data package will comprise a training sequence, also referred as pre-amble or pre-amble code, however, the training sequence is an overhead for data transmission. As for adaptive equalizer, different algorithms for updating coefficient need different lengths of training sequence, for example, the length of training sequence required by recursive least square RLS algorithm is less than that of the least mean square LMS algorithm, but the former needs more calculation and is more complex than the latter.
Another method for reducing the length of training sequence is the pre-loading technique of equalizer coefficient, which first writes a group of initial coefficient value pre-calculated (estimated) into the coefficient register of equalizer before the equalizer starts training process, if the initial coefficient value of equalizer is estimated accurately enough, the equalizer will nearly enter into the convergence state before training.
U.S. Pat. No. 5,970,092 “adaptively equalized burst receiver and method for upstream broadband data” introduced a method for equalizing the burst of uplink broadband data, this method uses Newman-Holfman sequence to estimate wireless channel approximately, then pre-loads the approximation of the channel estimation into the coefficient register of equalizer by using approximating method. In this method, because the approximation of channel estimation is used, meanwhile, the coefficient of equalizer is also pre-loaded by using approximating method, the equalizer can not be convergent enough in one burst, thereby interfering the performance of the system.